The technical field of this invention concerns analytical devices and methods for determining the presence of a bindable target substance (i.e. analyte) in a liquid sample possibly containing such substance. Various types of analytical devices, and methods employing the devices, have been used for this purpose. Many of these devices employ reaction membranes onto which a receptor capable of specifically binding to the target substance, is immobilized. In the assay that employs these types of devices, typically the sample to be tested is applied to the reaction membrane. If target substance is present in the sample, it binds to the immobilized receptor. Various methods are used to determine whether the target substance has bound to the receptor, thus indicating its presence in the sample. For immunoassays, where the target substance is an antigen, it is common to use antibodies that are capable of specifically binding to the antigen and that are labeled with detectable markers. When the labeled antibody is added to the reaction membrane, it will bind to the target antigen, if present, and the marker (e.g. fluorescent label, colored reagent, detectable enzyme marker, etc.) is detected.
Membrane-based analytical assays and devices have greatly simplified medical diagnostics. The results of a membrane-based analytical assay can be obtained in a matter of minutes. Quantitative results can be provided by special instruments designed to read the test results. Various types of membrane-based analytical assays are described in U.S. Pat. No. 5,006,464 to Chu et al., U.S. Pat. No. 4,818,677 to Hay-Kaufman et al., and U.S. Pat. No. 4,632,901 to Valkirs et al., and U.S. Pat. No. 5,185,127 to Vonk et al. There is a continued need to further simplify membrane-based analytical assays and to provide more rapid and more sensitive assays.
The invention is directed to analytical devices, methods of assembling the analytical devices and assays employing the analytical device for use in the detection of a target substance in a liquid sample. The analytical device comprises a surfactant-treated porous reaction membrane having an exposed sample-contacting surface and at least one receptor area located in a limited region of the exposed sample-contacting surface. The limited region has a higher concentration of surfactant than areas of the sample-contacting surface that are peripheral to the limited region. The analytical device is prepared by assembling an analytical device comprising a porous reaction membrane having an exposed sample-contacting surface, applying a surfactant-containing solution comprising at least 0.2% surfactant to the reaction membrane and allowing it to dry, and applying a receptor reagent to a limited region of the reaction membrane before, after, or at the same time as the surfactant-containing solution is applied to the reaction membrane. In performance of the assay, the surfactant causes the liquid sample to flow faster through the portion(s) of the reaction membrane where receptor molecules are located, relative to areas of the reaction membrane where receptor molecules are not located, thereby increasing assay sensitivity. In one embodiment of the invention, the surfactant is applied to the same limited region of the reaction membrane where receptor molecule is, or will be applied.